1. Field of the Invention
The present invention relates to a high-strength zinc base alloy, and particularly to a zinc base alloy which has high mechanical strength, a low casting temperature and a good fluidity, and which is suitable for use in molds and die casting.
2. Description of the Prior Art
It is thought that zinc base alloys may generally be used in molds and die casting.
Zinc base alloys for molds are first described below. It is known that the zinc base alloys may be employed in experimental molds utilizing the good casting properties thereof. Such experimental molds are generally used for experimentally manufacturing, for example, injection-molded products or sheet metal workpieces of automobile parts, and are thus distinguished from general-type molds used for mass-producing articles. In view of the need to ensure that each of the experimental molds used has a proper degree of strength, can be formed in a short time and is low-priced, experimental molds are manufactured by sand mold casting into shapes which substantially need not be subjected to cutting and are similar to the final shape required in each case and are then subjected to finishing polishing. Most of such zinc base molds are presently made of ZAS alloy (trade name; Al, 3.9 to 4.3%; Cu, 2.5 to 3.5%; Mg, 0.03 to 0.06%; balance; Zn). This ZAS alloy is suprior to other alloys with respect to its good pattern reproducibility, mechanical strength and the ease with which it is subjected to melt casting.
Iron base molds which are obtained by cutting and grinding a large steel forged block are used as general-type molds. Such iron base molds have such high strength that the can withstand several hundreds of thousands of shot operations, but they involve a long delivery time and are high-priced.
However, since there has been a recent tendency to produce many kinds of articles in small amounts, the use of a conventional steel mold which involves a long delivery time and has a high cost raises the effective cost of each mold that has to be borne by each product. There has therefore been a demand for the appearance of a mold which can be produced easily and is low-priced. There is a strong demand for a zinc base alloy that can be applied to a mold for mass-producing which can withstand five hundred thousand shot operations. Thus, various proposals with respect to zinc base alloys that can be used for molds have been made. If a ZAS alloy of the type known to be generally used for experimental molds is used for the above-described purpose without modification, the ZAS alloy mold obtained is slightly short of strength and fails to display the strength needed to withstand several hundred thousand shot operations. Although various types of improved zinc base alloys have therefore been produced by way of experiment with a view to increasing the strength of ZAS alloy, none of these alloys has been able to solve the problem that the low casting temperature and excellent flow properties which are the merits of a ZAS alloy have to be sacrificed to some extent.
At present, two types of zinc base alloys for die casting are specified by JIS. It is thought that, of these two alloys, an overwhelmingly large amount of the zinc base casting alloy class 2 (ZDC2) is used and accounts for 95% of the total amount of zinc base alloy used. This ZDC2 is an alloy composed of 3.9 to 4.3 wt% of Al, 0.03 to 0.06 wt% by of Mg, substantially all the balance being Zn. It has been used for about 35 years, and is widely utilized in mechine parts, decorative parts and articles for daily needs. This ZDC2 is characterized by the advantages that hot chamber die casting is possible because it has a long mold life, an appropriate mechanical strength, is readily machined and easily plated.
However, in recent times, the fields in which Zn die castings are employed have has been increasingly narrowed with the advent of plastic materials and Al die castings the qualities of which have been improved to a remarkable extent. There has therefore been a demand for the appearance of a new Zn base alloy for die casting which can expand the market for Zn die castings and which not only has high strength but may also be made thin. To this end, several improved Zn base alloys have been developed. However, none of these improved Zn base alloys has solved the problem that the low casting temperature and good fluidity which are the merits of Zn base alloys are sacrificed to some extent because the strengths thereof are given priority over other their properties.